1. Field of the Invention
This invention relates generally to the regeneration of ion exchange resins. More particularly, this invention involves the regeneration of anion and cation exchange resins which have been used for the removal of silica and various mineral salts from water.
2. Description of the Prior Art
Silica is present in virtually all natural waters, typically in amounts ranging from about 1 to 200 milligrams per liter or more. While silica is essential to the functioning of many biological organisms, including man, its present in water is known to create difficulties in many industrial operations. For example, in the steam generation facilities associated with most power generating plants, silica enters the boiler with the feed water and becomes concentrated therein due to the generation of steam. More importantly, however, silica also will be volatilized such that it is present in the steam in volatile form. Eventually the concentration of silica becomes sufficient to cause deposition of a hard scale in the boiler and/or in downstream equipment (particularly turbine blades) such that the boiler must periodically be purged of the silica concentrated water and replaced with fresh feed water. Such periodic purging and replacement operations are generally known in the industry as "blow-down." Since the frequency of blow-down as well as the quantity of water required to purge the boiler is dependent upon the amount of silica in the feed water, it is desirable to have as low concentration of silica in the boiler feed water as is possible. In this way, both water and energy usages can be minimized.
One method for removing silica as well as various mineral salts from water is by direct contact with a substantially homogeneous mixture (i.e., an intimate mixture) of anion and cation exchange resins in a mixed bed demineralizer wherein the anions and cations forming the salts are exchanged. This direct exchange continues until the exchange capacity of the resins is considered to be exhausted. Normally, this occurs when the resins become saturated with ions absorbed from the water. It is then necessary to restore the exchange capacity by regeneration.
In typical ion exchange processes, the substantially homogeneous mixture of anion and cation exchange resins is separated into two discrete layers prior to regeneration. This is usually accomplished by passing water upward through the demineralizer (often termed "backwashing") such that the admixture of anion and cation exchange resins stratifies into two discrete layers due to the different densities of the anion and cation exchange resins. Usually the anion exchange resin occupies the upper layer while the heavier cation exchange resin collects in the lower layer.
After the so-called "hydraulic grading" of the mixed bed as described above, regeneration of the exchange resins is normally effected by first passing an anion regenerant solution through the anion exchange resin and then a cation regenerant solution through the cation exchange resin (see Applebaum, Samuel B., "Demineralization by Ion Exchange", Academic Press, New York, 1968 and Arden, T.V., "Water Purification by Ion Exchange," Plenum Press, New York, 1968, the disclosures both of which are incorporated herein by reference). It has also been suggested to wash both the anion and the cation exchange resin with the anion regenerant solution prior to regenerating the cation exchange resin (see U.S. Pat. Nos. 2,666,741 and 2,736,698).
However, these methods of regenerating the anion and cation exchange resins are not effective in reducing the concentration of silica in the treated water to ultralow levels; i.e., concentrations of less than 0.01 milligrams per liter as SiO.sub.2. Thus, in view of the advantages of minimizing the amount of silica in water employed in industrial applications, it would be desirable to have available a simple and convenient method for reducing the concentration of silica to levels below those presently obtainable.